Underwater deployment and trenching of cable are nowadays common procedures practiced by many operators for instance when permanently installing optic, power and/or seismic cables offshore. Typically a cable is continuously deployed and submerged into the water column from a surface vessel such as a barge, a DP vessel or any other ship, pontoon or platform. A remotely operated underwater vehicle (ROV) at the ocean bottom is maneuvered from the vessel and is used for burying the cable into the seabed.
Seismic cables are typically designed as one long cable being interrupted by numerous sensor stations/sensor points for instance geophones or hydrophones for instance intended for seismic surveillance of deep water hydrocarbon reservoirs.
For cables in general, and seismic cables in particular, it is very challenging to deploy cable from the vessel all the way to the ocean floor without exposing it to too much mechanical strain which could damage the structure and components. This is especially a problem at deep waters where the distance from the surface to the seabed may be several thousand meters which of course leads to very high loads (e.g. from water currents, turbulence and/or vessel heave) acting on the cable hanging from the deck. It has therefore been suggested to arrange the cable on a reel and/or inside a cage prior to lowering it into the water and onto the ocean bottom whereat the cable is unwound and trenched e.g. by a ROV. However such a solution brings a risk that the cable will get tangled during unwinding, and/or that the seismic stations get trapped in between layers of cable and get damaged.
U.S. Pat. No. 6,350,085 discloses a cable deployment system wherein a section of a cable intended to be trenched is placed inside a drum, and another section of the cable act as a riser cable section whereby which the drum may be submersed from the vessel to the sea floor. The drum forms part of an integrated transportation and deployment system and is fitted onto a stinger for conducting the cable from the drum onto the sea bottom.
Another system and method for deploying and retrieving a cable is shown in US 2005/0276665 where the cable is wound upon a reel positioned inside a cage, which cable is arranged to be buried by a ROV.
However, also these two concepts will only reduce the problem, not eliminate it.